Dear Anna,

I realise you've already received many suggestions - here are two more.

Firstly, you may be able to use the anomalous signal of the metal to
locate the particle, even if it is disordered - have a look at the
following paper, describing the use of the anomalous signal of Xenon
to "see" the detergent/lipid structure within membrane protein
crystals (http://scripts.iucr.org/cgi-bin/paper?gr2184).

Secondly, perhaps electron microscopy or tomography may be able to
help you - transmission electron micrographs of different planes
through a crystal may give you some
idea of periodicity and disorder, as the nanoparticle should stand out
with very high contrast, given it is much more electron rich than the
surrounding protein lattice.

Good luck!

Cheers,
Oliver Clarke.

On 7 May 2012, at 17:30, anna anna <[log in to unmask]> wrote:

> Dear all,
> I'd like some suggestions/opinions about the sense of an experiment propos=
ed by a collaborator expert in saxs.
> In few words, he wants to collect SAXS data on a suspension of protein xta=
ls to investigate "low resolution periodicity" of the xtal (more details bel=
ow).=20
> The experiment requires a very huge number of xtals to obtain the circles t=
ypical of saxs and it is very time-consuming to me (I know nothing about sax=
s, I have only to prepare the sample). I proposed to measure a single rotati=
ng xtal (like in XRD) but he told they don't have a goniometer on saxs beaml=
ine.
> Here is my concern: does it make sense to measure many xtals together? Don=
't we lose information with respect to single xtal? And, most of all, what c=
an I see by saxs that I can't see by waxs??
> Sorry for the almost off-topic question but I think that only someone who k=
nows both the techniques can help me!!
>=20
>=20
> Some detail for who is intrigued by my story:
> we prepared doped magnetite nanoparticles using ferritin as bioreactor. I c=
rystallized this spheres filled with metal and solved the structure at 3.7A b=
ut I can see only the protein shell while there is no density inside, even i=
f I know that the nanoparticles are there. A simple explanation is that the p=
articles are free to move in the cavity(note that the diameter of the nanopa=
rticle is shorter then the inner diameter of the protein shell), ie are diso=
rdered, and do not contribute to diffraction, in fact, to my knowledge, nobo=
dy have ever seen the metal core inside ferritin or dps proteins. However, s=
ince they are magnetic particles they must "see" each other through the prot=
ein wall, ie they can't be completely free to move in the cavity. Maybe, but=
 this is just my opinion, I don't see the particle because the "period of th=
e particle" in the xtal is different/longer than the period of the protein s=
hell.
> Anyway, we are interested in the relative distance between the magnetic pa=
rticles in the xtal to study the effects of magnetostatic interactions in na=
noparticles 3D arrays. We are going to do this by saxs since, they told me, l=
ower resolution is useful in studying this long range periodicity (the diame=
ter of ferritin is about 120A) but it seems fool to me using a suspension of=
 so many xtals to obtain a scattering curve while I could collect diffractio=
n images from a single xtal!!! I know that saxs is used when you don't have x=
tals but if you have xtals, ie your system is ordered, xtallography is much m=
ore powerful!!
>=20
> Another question: how can I handle my diffraction data at 3.7A resolution t=
o "look for" nanoparticles? Should I try a lower symmetry? Maybe the anomalo=
us signal? Have you any reference for a similar case?
>=20
> Thank you very much!!
>=20
> anna
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